1. Field of the Invention
The present invention relates to a thermal shutoff circuit operable in response to a temperature, and more particularly to a thermal shutoff circuit for protecting an external circuit by shutting off a bias current to be supplied to the external circuit when the surrounding temperature rises above a prescribed level.
2. Description of the Prior Art
Conventionally, a thermal shutoff circuit, as shown in FIG. 1, is used together with an external circuit such as an amplifier circuit (not shown in the drawing). In FIG. 1, a first constant current source 13 and a Zener diode 14 are connected in series between a power supply terminal 11 with a voltage Vcc and a reference potential terminal 12 with a ground potential (GND). The voltage of a Zener diode varies in response to temperature, as is well known. The anode of zener diode 14 is connected to the base of an NPN transistor 15 for detecting a temperature change. Detection transistor 15 is connected at its collector to power supply terminal 11, and at its emitter to reference potential terminal 12 through a voltage divider comprised of a series circuit of resistors 16, 17. The voltage division node of the voltage divider, i.e., the connection node of resistors 16, 17 is connected to the base of an NPN transistor 18 for shutoff control, as described later. A second constant current source 19 and an NPN transistor 20 supply an external circuit, such as an amplifier circuit (not shown), with a bias current. The source 19 and the transistor 20 are connected between power supply terminal 11 and reference potential terminal 12 in series. Bias supply transistor 20 is connected in a diode fashion, by itself. That is, the collector and the base of bias supply transistor 20 are directly connected to one another. The collector of shutoff control transistor 18 is connected to the connection node of constant current source 19 and bias supply transistor 20. The base of bias supply transistor 20 is connected to an output terminal OUT for supplying the external circuit with the bias current.
In the conventional circuit, as described above, a first constant current I13 is produced by first constant current source 13 and flows into zener diode 14. This produces a zenor voltage Vz across zener diode 14. Zener voltage Vz has prescribed temperature characteristics, so that it varies in accordance with temperature, as described in detail later. Zener voltage Vz is applied to the base of detection transistor 15. A current flowing through detection transistor 15 varies in accordance with zener voltage Vz. Thus detection transistor 15 detects temperature change by the variation of its current. The detection result is obtained as a potential change on the emitter of transistor 15. The emitter potential of detection transistor 15 is divided by the voltage divider of resistors 16, 17 so that a prescribed voltage, i.e., a voltage across resistor 17 is given on the base of shutoff control transistor 18. The voltage is applied to the base of shutoff control transistor 18 and is referred to as Vb18 hereinafter.
Second constant current source 19 produces a second constant current I19. When constant current I19 flows into bias supply transistor 20, bias supply transistor 20 operates to draw the bias current as its base current from the external circuit through output terminal OUT. At that time, a base potential of a prescribed level exists on the base of bias supply transistor 20, i.e., on output terminal OUT. When constant current I19 fails to flow into bias supply transistor 20, i.e., constant current I19 flows into shutoff control transistor 18, as described in detail later, bias supply transistor 20 fails to draw the bias current from the external circuit. At that time, the external circuit is shut off.
Base potential Vb18 of shutoff control transistor 18 is expressed by the following equation. ##EQU1##
In this equation, Vb15 is the base-to-emitter voltage of detection transistor 15, and R16 and R17 are the resistances of resistors 16, 17, respectively. Vbe generally represents the base-to-emitter voltage of a transistor when the transistor is activated.
As is well known, the zener voltage V2 of a zener diode has a positive temperature characteristic, while the base-to-emitter voltage Vbe of a transistor has a negative temperature characteristic. In the equation (1), therefore, base potential Vb18 of shutoff control transistor 18 has a positive temperature characteristic. In other words, base potential Vb18 of shutoff control transistor 18 increases as temperature rises.
The base-to-emitter voltage Vbe18 of shutoff control transistor 18 has a negative temperature characteristic similar to base-to-emitter voltage Vbe15 of detection transistor 15, mentioned above. That is, base-to-emitter voltage Vbe18 of shutoff control transistor 18 decreases as the temperature rises.
As an example, assume that resistances R16, R17 of resistors 16, 17 are set so that both base potential Vb18 and base-to-emitter voltage Vbe18 of shutoff control transistor 18 agree with each other at a prescribed temperature T1 higher than a normal temperature Tn. Thus, Vb18(T1)=Vbe18(T1) at temperature T1. In this state, shutoff control transistor 18 is deactivated at normal temperature Tn. This is because base potential Vb18(Tn) of shutoff control transistor 18 at temperature Tn is lower than base potential Vb18(T1) at temperature T1, while base-to-emitter voltage Vbe18(Tn) of shutoff control transistor 18 at normal temperature Tn is higher than base-to-emitter voltage Vbe18(T1) at temperature T1. In other words, base potential Vb18(Tn) is below the level required to activate shutoff control transistor 18, i.e., the prescribed base-to-emitter voltage Vbe18(Tn). Therefore, second constant current I19 from second constant current source 19 flows only into bias supply transistor 20, and not into shutoff control transistor 18. As a result, bias supply transistor 20 draws the bias current from the external circuit through output terminal OUT. Therefore, the thermal shutoff circuit supplies the external circuit with the bias current at temperature Tn.
When temperature goes up to another prescribed temperature T2 above temperature T1, shutoff control transistor 18 is activated. This is because base potential Vb18(T2) of shutoff control transistor 18 at temperature T2 is higher than base potential Vb18(T1) at temperature T1, while base-to-emitter voltage Vbe18(T2) of shutoff control transistor 18 at temperature T2 is lower than base-to-emitter voltage Vbe18(T1) at temperature T1. In other words, base potential Vb18(T2) is sufficient to activate shutoff control transistor 18 at temperature T2. Therefore, second constant current I19 flows into shutoff control transistor 18. At this time, bias supply transistor 20 is deactivated due to the shortage of current flowing therethrough. As a result, bias supply transistor 20 fails to draw the bias current from the external circuit through output terminal OUT. That is, the thermal shutoff circuit shuts off the supply of the bias current and protects the external circuit from thermal breakdown, when the surrounding temperature exceeds the prescribed temperature T1.
However, in the conventional thermal shutoff circuit shown in FIG. 1, zener diode 14 is used as a voltage source which varies in response to temperature. Zener diodes, however, generally have zener voltages as high as 7 volts. As a result, the conventional thermal shutoff circuit, as shown in FIG. 1, requires a very high power supply voltage Vcc above the zener voltage, e.g., at least 8 volts. The conventional thermal shutoff circuit, therefore, is inappropriate for use in battery driven apparatus. Moreover, the conventional thermal shutoff circuit has a drawback in that it consumes a relatively large amount of power due to the high power supply voltage.
A second conventional thermal shutoff circuits, as shown in FIG. 2, is an improvement over the first conventional thermal shutoff circuit shown in FIG. 1. The differences between the first and second conventional thermal shutoff circuits will be described in detail hereinafter. In FIG. 2, a so-called V.sub.T referenced type constant current source 21 is used as the source for temperature responsive variable voltage. V.sub.T referenced type constant current source 21 is comprised of three PNP transistors 22, 24, 26, two NPN transistors 23, 25 and two resistors 27, 28. PNP transistors 22, 24, and 26 are connected with each other in the form of a current mirror circuit. That is, their bases are connected together and their emitters are connected to power supply terminal 11. Further, one PNP transistor, e.g., PNP transistor 24 is connected in diode fashion. NPN transistors 23, 25 also have their bases connected together. One NPN transistor, e.g., NPN transistor 25 is connected directly at its emitter to reference potential terminal 12. NPN transistor 23 is connected in diode fashion to itself and its emitter is connected to reference potential terminal 12 through resistor 27. The diode fashion NPN transistor, i.e., NPN transistor 23 is connected at its collector to the collector of PNP transistor 22. NPN transistor 25 is connected at its collector to the collector of the diode fashion PNP transistor 24. NPN transistor 23 has an emitter area N times larger than NPN transistor 25, N being a number larger than 1 (N&gt;1). PNP transitor 26 is connected at its collector to reference potential terminal 12 through resistor 28. The rest of the circuit shown in FIG. 2 is equivalent to the circuit shown in FIG. 1, i.e., the first conventional thermal shutoff circuit. For example, PNP transistor 26 is connected at its collector to the base of detection transistor 15.
As is well known V.sub.T referenced type constant current sources generate a voltage which varies in response to temperature. The voltage generated in V.sub.T referenced type constant current source 21 will be referred as thermal voltage Vt hereinafter and can be expressed by the following equation. ##EQU2##
In this equation, K represents the Boltzman's constant, T represents the absolute temperature and Q represents the electron charge.
Therefore, a current I27 expressed by the following equation flows through resistor 27. ##EQU3##
In this equation, R27 is a resistance of resistor 27.
An equivalent current flows through PNP transistor 26 in the current mirror circuit. This current, therefore, flows into resistor 28, so that a voltage V28 exists across resistor 28 and is applied to the base of detection transistor 15. Voltage V28 is expressed by the following equation. ##EQU4##
In this equation, R28 is the resistance of resistor 28 and Kr is a constant representing the ratio of resistance R28 to resistance R27.
Base potential Vb18 of shutoff control transistor 18 can be expressed by the following equation. ##EQU5##
In this equation, V17 is the voltage across resistor R17.
Voltage V28, therefore, has the same temperature characteristic as thermal voltage Vt obtained in V.sub.T reference type constant current source 21. As a result, the second conventional bias shutoff circuit shuts off the supply of the bias current to the external circuit and protects the external circuit from thermal breakdown when the temperature exceeds a prescribed temperature T1.
The second conventional thermal shutoff circuit shown in FIG. 2 has merit in that it operates at a low power supply voltage. However, the prescribed temperature at which the circuit operates to shut off the bias current varies in different integrated circuits. This is because a factor determining the prescribed temperature, i.e., the base-to-emitter voltage Vbe of the transistors, is a function of the current amplification ratio .beta. of the transistors as expressed by the following equation. ##EQU6##
In this equation, Ic is a collector current of the transistors, Is is the saturated current of transistors and Ib is the base current of transistors.
As is well known, the current amplification ratio .beta. varies in every different circuit device such as an integrated circuit chip. The current amplification ratio varies over a wide range, for example, from about 70 to about 300. Therefore, in the second conventional thermal shutoff circuit, the prescribed temperature differs over a wide range and it is not feasible to use such a shutoff circuit with different integrated circuits.
The same drawback also occurs in the first conventional thermal shutoff circuit shown in FIG. 1. That is, the circuit of FIG. 1 also uses the base-to-emitter voltage Vbe as a factor for determining the prescribed temperature.